Pure silica core, high birefringence, single polarization optical waveguide

ABSTRACT

Methods and apparatus provide for birefringent waveguides suitable for optical systems exhibiting polarization dependence such as interferometer sensors including Sagnac interferometric fiber optic gyroscopes (IFOG). The waveguides, for some embodiments, may offer single polarization performance over lengths of about a kilometer or more due to polarization dependent attenuation. According to some embodiments, the waveguides incorporate a pure silica core for resistance to radiation-induced attenuation (RIA).

GOVERNMENT RIGHTS IN THIS INVENTION

This invention was made with U.S. government support under contractnumber N00173-04-C-6024. The U.S. government has certain rights in thisinvention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the invention generally relate to optical waveguidestructures for propagating light signals in a single polarization and tosuch structures and associated devices for employment in radioactiveenvironments.

2. Description of the Related Art

Many optical components, such as fiber interferometric sensors, whichmeasure the phase change due to optical path length changes in fiberoptic implementations of Michelson, Mach Zehnder, Fabry-Perot, or Sagnacoptical interferometers, require use of single mode optical fiber andwaveguide devices. Such fibers may form components (e.g., theinterferometers themselves, fiber gratings and optical fiber couplers)for use with various optical transmission or measurement devices such asinterferometric fiber optic gyroscopes (IFOG). While only the lowestorder bound mode can propagate with conventional single mode fiber, thislight being guided may consist of a pair of orthogonally polarizedeigenmodes such that cross-coupling between polarizations can causeinterference and phase noise that can degrade sensor sensitivity andperformance.

Optical systems which exhibit polarization dependence thus may requireuse of polarization maintaining (PM) optical fibers to reducepolarization cross-coupling. The PM optical fibers maintain thepolarization state of polarized light signals launched into the fiber asthe signals propagate through the length of the fiber due tobirefringence of the fibers. However, cross-coupling still occurs inmany applications with the PM optical fibers especially when coiling andpackaging long lengths of the fiber for example in an IFOG, whichintroduces significant bending and mechanical perturbations that, alongwith any temperature fluctuations, promote cross-coupling betweenpolarizations.

While maintaining polarization reduces occurrence of cross-coupling,some fibers operate to remove or eliminate any cross-coupled orotherwise unwanted polarization states to promote single-polarizationoperation. Performance, design, expense and manufacturabilityshortcomings of prior single polarization or polarizing fibers precludeuse of these fibers in many operations and/or applications longer than afew meters. Examples of single polarization fibers include fibers havingelliptical-shaped cores or lossy cladding regions to promote adifference in attenuation between polarization modes that achievesextinction of one polarization mode over fiber lengths of a few metersor less. However, guided polarization mode attenuation also occurs as aside effect limiting applications to fiber lengths of a few meters orless, such as in a polarizer filter or pigtail, since longer lengthstend to produce unacceptable low intensity levels of even the guidedpolarization mode that is to be measured or otherwise used. Other singlepolarization fibers utilizing more conventional core/cladding designsrely on differences in fundamental mode cutoff wavelength betweenpolarization modes for single polarization operation. These fiberstypically operate over a narrow wavelength band that is highly sensitiveto fiber length, typically several meters, and the amount of bendingplaced on the length of fiber. This sensitivity results in limitedpackaging flexibility in achieving a desired polarization extinction.

In an exemplary application, a Sagnac interferometer may form an IFOGsensor constructed with long lengths (e.g., multiple kilometers) ofsensing fiber since sensitivity is proportional to the sensing fiberlength. However, increases in length of the fiber amplify undesiredpolarization effects that impair sensor performance, which is limited bysignal strength to phase noise (i.e., optical signal-to-noise ratio,OSNR) and is hence proportional to the amount of polarizationcross-coupling. In addition, applications of the IFOG sensor includenavigation systems employed in space and military operations whereionizing or nuclear radiation-induced attenuation (RIA) furthercontributes to signal loss and thus reduction in optical signal-to-noiseratio. Radiation resistant fibers include single mode designs thatpermit significant polarization cross-coupling. Problems associated withthe RIA and/or the polarization cross-coupling thwart attaining criticalperformance requirements and prevent ability to maintain design OSNR forthese IFOG sensors.

Therefore, there exists a need for improved methods and waveguides thatpropagate light signals in a single polarization. A further need existsfor such single polarization waveguides with improved resistance toradiation-induced attenuation to enable devices such as an IFOGutilizing the waveguide, to be employed in radioactive environments.

SUMMARY OF THE INVENTION

Embodiments of the invention generally relate to optical waveguidestructures. For some embodiments, a polarization maintaining, singlepolarization propagating, optical waveguide includes a central coreforming a light guiding path, an inner cladding layer surrounding thecore and having a refractive index lower than the core, a stress regiondisposed around the inner cladding layer and defining in cross sectionan elliptical outer shape, wherein the stress region induces strainbirefringence of the waveguide, an outer cladding layer surrounding thestress region, and a substrate layer disposed outside the outer claddinglayer. In some embodiments, a method of forming a polarizationmaintaining, single polarization propagating, optical waveguide includescreating a preform having a core, an inner cladding layer surroundingthe core, a stress region, an outer cladding layer surrounding thestress region, and a substrate layer disposed outside the outer claddinglayer, wherein an outer surface of the preform has a non-circular crosssection, and drawing the preform to produce the waveguide, wherein thedrawing rounds the outer surface and makes the stress region assume incross section an elliptical outer shape and induce strain birefringenceof the waveguide.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the presentinvention can be understood in detail, a more particular description ofthe invention, briefly summarized above, may be had by reference toembodiments, some of which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate onlytypical embodiments of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

FIG. 1 is a schematic end view of an optical fiber according toembodiments of the invention.

FIG. 2 is a plot of polarized spectral attenuation for the fiber shownin FIG. 1.

FIG. 3 is a graph of refractive index profiles across both a major axisand a minor axis of the fiber shown in FIG. 1.

FIG. 4 is a schematic end view of a preform utilized to manufacture thefiber shown in FIG. 1.

FIG. 5 is a schematic end view of the preform after shaping an outersurface of the preform to have a non-circular cross section and prior todrawing the preform, thereby producing the fiber shown in FIG. 1.

FIG. 6 is a sensing system utilizing optical fiber, according toembodiments of the invention, such as the fiber shown in FIG. 1.

DETAILED DESCRIPTION

Embodiments of the invention relate to birefringent waveguides suitablefor optical systems exhibiting polarization dependence such asinterferometer sensors including Sagnac interferometric fiber opticgyroscopes (IFOG). The waveguides, for some embodiments, may offersingle polarization performance over lengths of about a kilometer ormore due to polarization dependent attenuation. According to someembodiments, the waveguides incorporate a pure silica core forresistance to radiation-induced attenuation (RIA).

FIG. 1 illustrates an end view of an optical fiber 100 defined by a core102, an inner cladding 104, a stress region 106, an outer cladding 108,and a substrate layer 110. While depicted with reference to fiberoptics, any optical waveguide structure may benefit from configurations,properties and structures attributed to the fiber 100 as describedherein. The fiber 100 features the core 102 constituting pure silica andthe claddings 104, 108 made of, for example, fluorinated silica. Thesecore and cladding glasses have demonstrated improved resistance to RIA,which makes the fiber 100 suitable for long-term operation in spaceradiation environments or other radioactive environments, as well asbenign environments where applications lack significant exposure of thefiber 100 to radiation.

The core 102 defines an at least substantially circular cross sectionsurrounded by the inner cladding 104 that delineates an at leastsubstantially round annular shaped cross section. Doping with, forexample, fluorine (F) provides refractive index lowering of thecladdings 104, 108 relative to the core 102. This difference inrefractive index facilitates guiding light input into the fiber 100along the core 102. Further, doping with, for example, germanium (Ge)and boron (B) of the stress region 106 produces a refractive index ofthe stress region 106 that is also lower than the core 102 but at leastclose to and/or higher than the claddings 104, 108, which may havesubstantially similar refractive indices for some embodiments.

The stress region 106 disposed between the inner and outer claddings104, 108 makes the outer cladding 108 assume an asymmetric shape such asan elliptical band shaped cross section during manufacturing of thefiber 100. As a result of these shapes, the outer cladding 108 comes inclose proximity to, but not necessarily touching, the inner cladding 104in the direction of a minor axis 112 of the fiber 100. Further, theelliptical band shaped cross section of the outer cladding 108 distancesthe outer cladding 108 from the inner cladding 104 in the direction of amajor axis 114 of the fiber 100. The substrate layer 110 made fromsilica, for example, and having a refractive index similar to the core102 further surrounds the outer cladding 108 in a manner that forms anat least substantially circular outer circumference of the fiber 100.

Asymmetric shaping of the stress region 106 around the core 102 impartsstress-induced birefringence such that the fiber 100 tends to maintainthe polarization of the light input into the fiber 100 as the lightpropagates through the core 102 along the length of the fiber 100.Further, because the refractive indices of the stress region 106 andinner and outer claddings 104, 108 are substantially similar, theeffective optical cladding thickness is different between the axes 110,112 of the fiber 100 with the minor axis 112 substantially thinner. Thethinner effective optical cladding along the minor axis promotes leakyguidance and/or other factors such as absorption and light pullingstructures bestow a different attenuation rate between polarizations,thereby promoting single polarization operation of the fiber 100 overkilometer lengths of the fiber 100.

FIG. 2 graphs a polarized spectral attenuation for the fiber 100 toillustrate the different attenuation rate between polarizations forvarious wavelengths. Polarization eigenmodes propagating in the core 102oriented in alignment with the minor axis 112 attenuate according todropped curve 212 while polarization eigenmodes aligned with the majoraxis 214 experience loss according to guided curve 214. The guided curve214 plots major axis attenuation rate of the light at 1550 nanometers(nm) as being about 1.5 decibels per kilometer (dB/km). With referenceto the dropped curve 212, leaky mode and/or other factors attribute toabout 15.0 dB/km loss in light strength value of the minor axis at 1550nm. This attenuation rate difference between polarizations results insingle polarization operation of the fiber 100 with a −14.0 dBpolarization extinction for each kilometer of the fiber 100. Forexample, a 3.0 km sensing coil, which is suitable in length fornavigational-grade IFOG, formed using the fiber 100 provides −42.0 dBextinction and guided mode attenuation of 4.5 dB, thereby remaining wellwithin a power budget for these sensors. In other words, less than afraction of a percentage of any undesired cross-coupled light, ifpresent, remains after propagating a few kilometers or less within thefiber 100 while leaving detectable levels of the polarization statewanted for analysis.

FIG. 3 plots a refractive index profile across the major axis 114 of thefiber 100 represented by solid curve 314 and the minor axis 112 of thefiber 100 depicted as dashed curve 312. Since depressed-index claddingis subject to tunneling or leaky mode guidance, part of the mode thatoverlaps the claddings 104, 108 and stress region 106 can tunnel andleak light into the claddings 104, 108 and stress region 106 and beattenuated. Rate of this leaky mode attenuation depends on severalfactors including an effective cladding thickness provided by thecladdings 104, 108 and stress region 106 due to the refractive indicesof the claddings 104, 108 and stress region 106 compared to the core 102and the substrate layer 110. Visible in the solid and dashed curves 312,314 and the disposition of the substrate layer 110 within the fiber 100,the effective cladding thickness ends at about 20 micrometers (μm) inthe direction of the minor axis 112 yet extends further to about 35 μmalong the major axis 114.

In addition to the effective cladding thickness, the inner cladding 104,the stress region 106, and the outer cladding 106 define along the majoraxis 114 refractive indices (see, areas between about 5 μm and about 35μm) according to the solid curve 314 that provide substantiallyconsistent effective cladding properties to contain the light in thecore 102. By contrast, the inner cladding 104, the stress region 106 andthe outer cladding 106 possess in conformance with the dashed curve 312more differing refractive indices within a shorter distance (see, areasbetween about 5 μm and about 20 μm) in the minor axis 112 and henceinconsistent effective cladding properties. In particular, lowerrefractive indices associated with the inner cladding 104 and the outercladding 108 bound peaks 306 of the dashed curve 312 corresponding withthe stress region 106. Further, the peaks 306 occur in proximity (e.g.,about 5 μm) of the core 102 to enable pulling of light into the stressregion 106 along the minor axis 112 such that the stress region 106 insubstantially only the direction of the minor axis 112 acts as a partialannular cladding mode carrier that is lossy due to the stress region 106being a poor waveguide.

FIG. 4 illustrates a preform 400 utilized to manufacture the fiber 100.The preform 400 includes a core layer 402, an inner cladding layer 404,a stress region layer 406, an outer cladding layer 408, and an externalsubstrate tubing 410 that are all circular and arranged concentric toone another upon completion of deposition processes forming the preform400. For some embodiments, the core layer 402 contains at leastsubstantially pure silica (SiO₂). Deposition of the cladding layers 404,408 may occur in processes to provide the cladding layers 404, 408 withabout 15.0 mol % fluorine in silica. Additionally, silica may form theexternal substrate tubing 410.

Deposition processes produce the stress region layer 406 with sufficientdoping to change a thermal coefficient of expansion for the stressregion layer 406 relative to the core layer 402, the inner and outercladding layers 404, 408, and the external substrate tubing 410. Inaddition, the doping may adjust the refractive index of the stressregion layer 406 to at least approach matching the lower refractiveindex of the cladding layers 404, 408 relative to the core layer 402.Doping during deposition of the stress region layer 406 may produceabout 50.0 mol % dopants that may be selected from boron and germanium.For some embodiments, the stress region layer 406 contains about equalamounts of boron and germanium, which concentrations may be adjusted toraise or lower the refractive index of the stress region layer 406.

FIG. 5 shows the preform 400 after shaping an outer surface of thepreform 400 and prior to drawing the preform 400, thereby producing thefiber 100 shown in FIG. 1. The shaping produces a non-circular crosssection of the preform 400. For some embodiments, the external substratetubing 410 may include lengthwise extending first and second parallelflattened surfaces 502, 504 to form the non-circular cross section ofthe preform 400. Other embodiments include the external substrate tubing410 having likewise lengthwise extending parallel first and secondmachined surfaces but with concave machined surfaces to impart a more“peanut” shaped cross section, and pairs of lengthwise extendingparallel surfaces to provide for a diamond shaped cross section.

Control of subsequent draw temperature and draw speed of the preform 400to make the fiber 100 ensures the fiber 100 takes the shape andconfiguration described heretofore. During the drawing of the preform400, the different thermal coefficient of expansion of the stress regionlayer 406 results in the stress region layer 406 having a relativelylower viscosity (i.e., more fluid) compared to the core layer 402, theinner and outer cladding layers 404, 408, and the external substratetubing 410. Once heated, the outermost surface of the external substratetubing 410 assumes a shape based on surface tension interactionscorresponding with the least energy meaning that the outermost surfaceof the external substrate tubing 410 goes to a circular shape. Thisrounding of the first and second parallel flattened surfaces 502, 504 orother shaped surfaces enables the external substrate tubing 410 toachieve a substantially circular outer circumference and causes materialdisplacement compensated for by the stress region layer 406 flowingtoward an elliptical outer shape due to the viscosity difference.Further, the difference in thermal coefficient of expansion of thestress region layer 406 introduces the strain birefringence since thestress region layer 406 that is last to harden becomes confined by theexternal substrate tubing 410 restricting natural contraction as thestress region layer 406 cools and later hardens.

FIG. 6 illustrates a sensing system 600 utilizing optical waveguidessuch as the fiber 100. The sensing system 600 includes a light source602, an interferometric sensor such as an IFOG formed from an opticalfiber sensing coil 604 that may contain between 200 m and 5.0 km offiber, and a detector 606. In operation, the light source 602 launchesinput light into the sensing coil 604 via transmission optical fibers610 connected by coupler 608. Rotation of the sensing coil 604 affectsthe input light, thereby generating response light signals. The responselight signals from the sensing coil 604 propagate through transmissionoptical fibers 610 to the detector 606 that then receives the responselight signals for measuring rotation of the sensing coil 604. For someembodiments, one or more of the sensing coil 604, the transmissionoptical fibers 610, and the coupler 608 incorporate the fiber 100 shownin FIG. 1.

While the foregoing is directed to embodiments of the present invention,other and further embodiments of the invention may be devised withoutdeparting from the basic scope thereof, and the scope thereof isdetermined by the claims that follow.

1. A polarization maintaining, single polarization propagating, opticalwaveguide, comprising: a central core forming a light guiding path; aninner cladding layer surrounding the core and having a refractive indexlower than the core; a stress region disposed around the inner claddinglayer and defining in cross section an elliptical outer shape, whereinthe stress region induces strain birefringence of the waveguide; anouter cladding layer surrounding the stress region, wherein the stressregion has a higher refractive index than the inner and outer claddinglayers; and a substrate layer disposed outside the outer cladding layer.2. The optical waveguide of claim 1, wherein the inner cladding has around annular shaped cross section.
 3. The optical waveguide of claim 1,wherein the inner and outer cladding layers are substantially alike incomposition.
 4. The optical waveguide of claim 1, wherein the core has acircular cross section.
 5. The optical waveguide of claim 1, wherein thecore consists of silica.
 6. The optical waveguide of claim 1, whereinthe core guides light of one polarization with less attenuation thananother polarization.
 7. The optical waveguide of claim 1, wherein thecore guides light at 1550 nanometers with attenuation at a firstpolarization less than 3.0 decibels per kilometer (dB/km) andattenuation at a second polarization more than 15.0 dB/km.
 8. Theoptical waveguide of claim 1, wherein the inner and outer claddinglayers are doped with fluorine.
 9. The optical waveguide of claim 1,wherein the substrate layer defines a circular outer circumference. 10.The optical waveguide of claim 9, wherein the substrate layer defines anoutermost external surface of the waveguide.
 11. The optical waveguideof claim 1, wherein the stress region has a different thermalcoefficient of expansion than the substrate layer.
 12. The opticalwaveguide of claim 1, wherein the stress region is doped with germaniumand boron.
 13. The optical waveguide of claim 1, wherein the substratelayer consists essentially of silica.
 14. The optical waveguide of claim1, wherein: the core consists of silica; the inner and outer claddinglayers are doped with about 15 mol % fluorine; and the stress region isdoped with about 50 mol % dopants selected from germanium and boron. 15.The optical waveguide of claim 14, wherein the substrate layer consistsessentially of silica.
 16. A method of forming a polarizationmaintaining, single polarization propagating, optical waveguide,comprising: creating a preform having a core, an inner cladding layersurrounding the core, a stress region, an outer cladding layersurrounding the stress region, and a substrate layer disposed outsidethe outer cladding layer, wherein an outer surface of the preform has anon-circular cross section and the stress region has a higher refractiveindex than the inner and outer cladding layers; and drawing the preformto produce the waveguide, wherein the drawing rounds the outer surfaceand makes the stress region assume in cross section an elliptical outershape and induce strain birefringence of the waveguide.
 17. The methodof claim 16, further comprising forming parallel flattened surfacesaround the substrate layer to provide the non-circular cross section.18. The method of claim 16, further comprising forming parallel concavemachined surfaces around the substrate layer to provide the non-circularcross section.
 19. The method of claim 16, further comprising formingpairs of parallel flattened surfaces arranged around the substrate layerto provide a diamond-shaped cross section.
 20. A sensing system,comprising: a light source; an interferometric sensor coupled to thelight source, wherein the sensor includes a polarization maintaining,single polarization propagating, optical waveguide, comprising: a coreforming a light guiding path; an inner cladding layer surrounding thecore and having a refractive index lower than the core; a stress regiondisposed around the inner cladding layer and defining in cross sectionan elliptical outer shape, wherein the stress region induces strainbirefringence of the waveguide; an outer cladding layer surrounding thestress region, wherein the stress region has a higher refractive indexthan the inner and outer cladding layers; and a substrate layer disposedoutside the outer cladding layer; and a sensor response signal detectorcoupled to the sensor.
 21. The sensing system of claim 20, wherein thesensor comprises a navigational-grade interferometric fiber opticgyroscope (IFOG) having multiple kilometers of the waveguide coiled intoa sensing coil.